The present invention relates to an improvement of a thermal head for a thermal printer, in particular, relates to such a head which operates in higher temperature and high power capacity, for providing clearer and rapid printing.
With the advent of computer technology and advances in the arts of data processing, data communication and/or a facsimilie communication, requirements for increased speed of information handling have become more stringent. One known type of rapid printing is a high speed thermal printer, which has at least a thermal head and a printing paper, and operates on the principle that a thermal head, heated to a high temperature according to the pattern of a desired character to be printed, selectively changes the color of a thermal paper. A thermal printer has the advantage that it can print not only a predetermined pattern of characters, but also any pattern desired including pictures, Chinese characters, and/or Arabian characters.
A thermal printer is a kind of a dot printer which composes the pattern to be printed with a plurality of dots, and a thermal head has a plurality of heat cells arranged, for instance, in a straight line for printing these dots. As the thermal paper moves in a direction perpendicular to said straight line of heat cells, said heat cells are selectively heated, thus the color of the thermal paper is selectively changed. Thus the desired pattern is printed on the thermal paper.
We have proposed some thermal head, U.S. Pat. No. 4,136,274 (UK Pat. No. 1,524,347) is one of them.
FIG. 1 is the cross section of a prior thermal head disclosed in said U.S. patent. In FIG. 1, the reference numeral 10 is a glazed alumina substrate having a glazed layer 15 with 40-80 .mu.m thickness, 30 is a heater layer with the thickness of 1000 .ANG. through 2000 .ANG. made of for instance tantalium nitride (T.sub.a2 N), 40 is a conductive layer attached to the heater layer 30 for providing the electrical coupling of the heater line with an external circuit, 50 is an S.sub.i O.sub.2 layer with the thickness of 1-3 .mu.m for preventing the oxidation of the heater line, and 60 is a protection layer for reducing the wear of heaters due to friction with a thermal paper, and said protection layer 60 is made of for instance T.sub.a2 O.sub.5 with the thickness of 3-10 .mu.m.
The structure of FIG. 1 has the advantages that the fluctuation of the resistance of a heater layer is small, and the life time of a head is so long as the power applied to the head is small, however, it has the disadvantage that the power capacity of a head is small. That is to say, a prior thermal head can not have much power capacity, and therefore, can not provide a high temperature. The operation at high temperature is essential for a high speed printing. For instance, the highest power consumption of a prior thermal head is up to 1.2 watts when the width of a heater layer is 110 .mu.m, the length of a heater layer is 215 .mu.m, the sheet resistance of a heater layer is 17 ohms/square, that heater layer is heated with a pulse signal with the pulse width 1 msec and the period 50 msec for 30 minutes. If that prior thermal head is heated with the power higher than 1.2 watts, that heater is broken.
FIG. 2 is an explanatory drawing of a sheet resistance, in which 30 is a rectangular heater with the side length L, and 100 and 102 are conductors with the width L. In that configuration, the resistance between conductors 100 and 102 is independent from the length L, but it depends solely upon the thickness of the heater 30 and the material of the heater 30. Therefore, the sheet resistance of the heater 30 is defined by the resistance between the conductors 100 and 102, and is expressed as R ohms/square, if the resistance between the conductors 100 and 102 is R ohms.